UTRAN (Universal Terrestrial Radio Access Network) is a term that identifies the radio access network of a UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) and NodeBs i.e. radio base stations. The NodeBs communicate wirelessly with mobile user equipments (UEs) and the RNCs control the NodeBs. The RNCs are further connected to the Core Network (CN). Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further, the E-UTRAN consists of eNodeBs (evolved NodeBs), and the eNodeBs are interconnected and further connected to the Evolved Packet Core network (EPC). E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP).
Network-initiated mobility is defined in the 3GPP-specification TS 36.331 as part of RRC (Radio Resource Control). The RRC is a concept and a protocol name for a set of control messages exchanged between the UE and the RNC in the LTE/E-UTRAN standard. When a UE is in RRC_CONNECTED mode, i.e. once the UE has an established RRC connection, the network controls UE mobility and decides when the UE shall move to which cell. The network triggers the handover procedure based on a variety of parameters, e.g. radio conditions, load, etc. To facilitate the triggering of the handover procedure, the network may configure the UE to perform measurement reporting. However, the network may also initiate handover blindly, i.e. without having received measurement information from the UE. More specifically, the network initiates the mobility to a UE in RRC_CONNECTED mode, possibly in response to the RRC message MeasurementReport from the UE, by sending the RRC message MobilityFromEUTRACommand
As part of RRC control, TS 36.331 specifies measurement configuration. Measurement gaps are periods that the UE may use when performing measurements i.e. no transmissions, neither uplink (UL) nor downlink (DL) transmissions, are scheduled during these periods. Moreover, the UE is always configured with a measurement gap pattern when inter-frequency or inter-RAT (Radio Access Technology) measurements need to be performed, i.e. when serving cell quality drops below a configured threshold value. The measurement gap pattern is characterized by a gap length, 6 ms or 8 ms, and by a gap period, 40 ms or 120 ms. During the measurement gaps, the UE does not need to monitor the PDCCH (Physical Downlink Control Channel) or any other downlink shared channels and the UE does not make any uplink transmissions on UL-SCH (Uplink Shared Channel). The network provides the measurement configuration applicable for a UE in RRC_CONNECTED state. Moreover, the network provides the measurement configuration by means of dedicated signaling i.e. on a signaling radio bearer. In particular, a number of measurement types are defined:                Intra-frequency measurements (measurements at the downlink carrier frequency of the serving cell);        Inter-frequency measurements (measurements at frequencies that differ from the downlink carrier frequency of the serving cell);        Inter-RAT measurements of UTRA frequencies;        Inter-RAT measurements of GERAN (GSM/EDGE Radio Access Network) frequencies;        Inter-RAT measurements of CDMA2000 HRPD (High Rate Packet Data) or 1xRTT (Radio Transmission Technology) frequencies.        
A measurement is configured with a reporting criterion and format:                Reporting criteria: The criteria that trigger the UE to send a measurement report. This can either be periodical or a single event description.        Reporting format: The quantities that the UE includes in the measurement report and associated information e.g. number of cells to report.        
When a measurement was triggered and the UE has performed the required measurement during the measurement gap, the UE assembles a measurement report and submit the MEASUREMENT REPORT message to lower layers for transmission on a Signaling Radio Bearer (SRB).
In the following the scheduling principles for LTE are described.
In LTE, the uplink MAC scheduler resides in the eNodeB and assigns transmission resources to UEs in the cell. In order to perform these tasks the scheduler needs information about the current buffer state of the UE i.e. whether and how much data the UE buffers in its priority queues. According to the existing framework for buffer status reporting for LTE, buffer status reporting is used by the UE to report to the eNB the amount of data stored in the buffers of the UE for transmission. The eNB uses these reports to allocate resources to the UE, and to prioritize resource allocation between different UEs.
Buffer Status Reports (BSR) and Scheduling Request (SR) are triggered when uplink data arrives in the UE transmission buffer and the data belongs to a radio bearer or a logical channel group with higher priority than those for which data already existed in the buffer. When new data arrives to the transmit buffers of a UE and the UE has no grant for transmission on the PUSCH (Physical Uplink Shared Channel), the UE need to request permission to transmit and consequently a BSR is triggered. If the UE has no PUSCH resource a SR is triggered as well. The SR will either be transmitted on the RACH (Random Access Channel), i.e. a RA-SR will be transmitted, or on dedicated resources on the PUCCH (Physical Uplink Control Channel, i.e. a dedicated SR (D-SR) will be transmitted, if such resources are available. The PUCCH resources for dedicated SR are assigned and revoked by the eNB through RRC.
In 3GPP the following assumptions regarding the behaviour of the UE for transmissions overlapping with the measurement gap have been agreed upon:                The UE does not retransmit on PUSCH in a subframe wherein it is configured to perform a measurement;        The UE shall drop certain UL transmissions overlapping with the measurement gap. The dropped UL transmissions include:                    PUSCH transmissions: transmissions, re-transmissions of persistent or scheduled allocations,            CQI (Channel-Quality Indicator) reports on PUSCH and PUCCH            Sounding Reference Signals (SRS);            SRs;            ACK: the measurement gaps are configured by the eNB, therefore the MAC scheduler resided in the eNB can make sure to avoid the overlapping of UE ACK transmissions and the measurement gaps.                        
Thus, the measurement gap takes priority over the configured PUCCH resource, in particular for SRs.
It has been observed that a problem can occur when the UE performs measurements and a SR opportunity collides with the measurement gap. The buffer status reporting may be delayed to an extent which could in the worst case lead to forcing the UE to drop the connection, i.e. go back to RRC_IDLE state. Thus, UE measurement performance at cell edge could be negatively impaired as measurements could be either not transmitted or not performed.